1. Field of the Invention
This invention relates to a motor control arrangement and in particular to a motor control arrangement for a brushless dc motor used in a centrifuge instrument.
2. Description of the Prior Art
Brushless dc motors are known in the art. Such motors are available from a wide variety of sources, such as, for example, Electric Indicator Company, Incorporated of Norwalk, Conn. Such a motor has a rotating element that carries an array of permanent magnets. A stationary armature formed of multiple phase windings surrounds the rotor. The windings are connected via a switching matrix to positive and negative busses. For a given direction of rotation the position of the permanent magnets on the rotor determines which of the switches are closed with respect to the positive and negative busses. The actual control signals which open and close the appropriate switches are generated by suitable current control and commutation control arrangements in a manner well known to those skilled in the art.
Typically these control elements are connected to the same voltage reference point and share common current paths with the motor. Thus, due to the normal commutation of the phases of the motor there occurs rapid switching of relatively high currents at relatively high voltages. Such conditions are exacerbated when a brushless dc motor is used to drive a centrifuge instrument because of the magnitude of the currents and voltages required to deliver the amount of power necessary to achieve the sustain centrifuge rotor speed. As a result an electrically noisy environment is created in which the control elements must operate.
In view of the foregoing it is believed advantageous to provide a control arrangement for a brushless dc motor used to drive a centrifuge instrument in which the control elements are isolated from the high noise environment.